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I just bought 25L of a product labelled:
Organic Mushroom Compost.

I know what I'm going to use it for, but am curious about the name.

Today alone, I've read (and been given/told) at least 3 contrasting explanations.
Which is it?


  1. Composted mushrooms.
  2. Compost for growing mushrooms.
  3. Compost that has already been used to grow mushrooms.
    (i.e: Recyclable byproduct of the mushroom-growing industry).
  4. Something else.
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"Mushroom Compost" is a product designed as a growth medium for mushrooms. However some people use it like other composts.

What is Mushroom Compost? (From OSU Extension)

"Often sold at landscape supply houses, mushroom compost can help amend garden soil, but should be used with caution, according to John Hart, soil scientist with the Oregon State University Extension Service. Mushroom compost is rich in soluble salts and other nutrients and can kill germinating seeds and harm salt-sensitive plants including rhododendrons and azaleas. The recipe for mushroom compost varies from company to company, but can include composted wheat or rye straw, peat moss, used horse bedding straw, chicken manure, cottonseed or canola meal, grape crushings from wineries, soybean meal, potash, gypsum, urea, ammonium nitrate and lime. The compost ingredients are weighed out, then mixed in."

I have seen the odd reference to Spent mushroom compost being referred to as simply "mushroom compost". Ideally, the compost you buy should either be in a fully labeled bag that indicates what it is and what you should use it for, or be offered in bulk by an individual who can tell you all that and more. If this information is unavailable, I wouldn't buy the product. Pathogens can be present in compost and often the "organic" label is tacked on only to increase the price even if it's not an approved organic fertilizer. Also keep in mind that spent mushroom compost isn't a great fertilizer. It should be used as soil conditioner, helping to build up your organics increasing the effectiveness of other fertilizers you use.

What is "Spent Mushroom Compost"? (From PSU Extension)

"Spent mushroom substrate still has some nutrients available for the mushroom; however, it is more economical to replace the substrate and start a new crop. Before removing the spent substrate from the mushroom house, the grower "pasteurizes" it with steam to kill any pests or pathogens that may be present in the substrate and casing. This final pasteurization kills weed seeds, insects, and organisms that may cause mushroom diseases. Users may consider spent substrate clean of weed seeds and insects."

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Were it spent mushroom compost, it should be called that - in the UK, it doesn't usually come in bags you can buy at the garden centre or wherever, spent mushroom compost is usually obtained from a mushroom grower, so I would assume, from your description, it's actually compost designed for growing mushrooms which hasn't been used before. Even if available at commercial outlets where you are, it should still be labelled as 'spent' mushroom compost if that's what it is.

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